1. Field of the Invention
The present invention relates to a color image forming apparatus which forms a color image.
2. Description of the Related Art
In recent years, a color image forming apparatus which is typified by, for example, a color printer and a color copying machine and adopts, for example, the electrophotographic or inkjet scheme must output higher-quality images. To meet this requirement, Japanese Patent Laid-Open No. 2003-084532 proposes a color image forming apparatus including a color sensor located downstream of a fixing unit. The color sensor irradiates a patch formed on a printing material with light to obtain its color value (color information) from the light reflected by it. The color image forming apparatus adjusts the tone of a toner image formed on the printing material, in accordance with the output from the color sensor.
Upon colorimetry of the patch formed on the printing material, the colorimetric value of the color information often varies depending on the temperatures of the printing material and toner. Namely, the colorimetric value of the heated patch immediately after fixing is different from that of the patch cooled to room temperature. This variation includes a variation due to the influence of a fluorescent material (for example, a fluorescent bleaching agent contained in the printing material) and that due to the influence of a nonfluorescent material (toner components), is commonly called thermochromism. Due to this thermochromism, the colorimetric value varies depending on the temperatures of the printing material and toner upon colorimetry of the patch output onto the printing material. Also, this variation exhibits different characteristics depending on the color of the patch. This generates an error in the colorimetric value when high-accuracy colorimetry is necessary. To reduce a measurement error due to thermochromism, the printing material heated upon fixing need only be cooled. However, when the apparatus is stopped until the printing material sufficiently cools, it takes a long time to perform one measurement operation. In other words, it is demanded to reduce a measurement error due to thermochromism while suppressing deterioration in usability.